1. Field of the Invention
This invention relates to communications networks, and has particular utility in relation to Passive Optical Networks (PONs).
2. Related Art
PONs are commonly used in point to multi-point communications applications, for example in a broadband local access optical network providing broadband services to residential and small business users. In a PON providing a point to multi-point local access optical network, a so called optical line termination unit (OLT) is provided, connected to the "head end" of the PON and located, typically, in a local telephone exchange. The OLT controls access to the shared capacity of the PON and interconnects the PON with a wider telecommunications network. So called optical network units (ONUs) are connected to the "user ends" of the PON, typically up to 10 km downstream from the OLT, to provide points of connection to users, to multiplex services and to communicate over the PON with the OLT.
Conventionally, data are passed over the PON between an ONU and the OLT according to a predetermined packet structure, data being multiplexed within that predetermined structure for the particular service being provided to a user. Data interchanges between OLT and ONU take place under the centralised control of the OLT using a predetermined protocol, designed to ensure that the available capacity of the PON is shared equitably between the connected ONUs and between the various services being carried. Data passing "upstream" over the PON from ONUs to the OLT are typically multiplexed according to a Time Division Multiple Access (TDMA) technique in which data channels are separated in time to avoid collision at the OLT. PONs employing a TDMA multiplexing technique are known as TDMA PONs. Various protocols, known as Media Access Control (MAC) protocols, have been developed to control an ONU's upstream access to the shared capacity on a PON. MAC protocols may implement the TDMA multiplexing scheme in the upstream direction, or they may implement other packet-based data transfer schemes more appropriate to especially high data rates or to a variable rate asymmetric data transport.
A typical "star" PON configuration does not permit ONUs to communicate directly with each other and to decide among themselves which one will next be allowed to send data upstream to the OLT. PON MAC protocols therefore tend to be controlled by the centralised OLT rather than that control being distributed amongst the ONUs.
Typically, MAC protocols implement two main types of control policy in the allocation of capacity for the upstream transfer of data packets from an ONU to the OLT--"Fixed Assignment" and "Demand Assignment" based upon ONU buffer fullness.
Under a fixed assignment policy a static allocation of capacity is made at the time of call set-up by a user via an ONU so that the same number of data packets may be sent from the ONU to the OLT each time the ONU's time slot arrives for transfer of its upstream data. In some time slots, not all the allocated capacity may be used by the ONU, while in others the ONU may have more packets in its buffer than the allocation will allow to be transmitted in that cycle, leading to packet delay. A demand assignment policy, on the other hand, allows the allocation of capacity to ONUs to vary according to demand, making more. efficient use of the PON.
A number of protocols have been developed incorporating the demand assignment policy, or variations on the theme. For example, with a polled pipeline protocol, demand for capacity by ONUs may be determined by the OLT by means of a process in which each ONU is polled in turn by the OLT for a specified maximum number of upstream data packets, the ONU including with any transferred data a message requesting that a specified number of data packets of upstream capacity be reserved for its next upstream slot during the next polling cycle. The capacity requested by the ONU is typically equal to the number of packets awaiting transmission in the ONU's buffer. Alternatively, the ONUs may send reservation messages to the OLT to increase or decrease capacity assigned, as and when required. In either case, the OLT makes a reservation for that ONU in an overall assignment schedule for the PON based upon the requested capacity, although allocations of capacity to a particular ONU by the OLT may be subject to ceilings during periods of heavy demand for capacity to ensure fair allocation across ONUs and to ensure that appropriate priority is given, for example, to low bit rate telephony services. In such circumstances, data packets can be delayed if they arrive in the ONU's buffer between the time of the last request for capacity by the ONU and the time of the ONU's next upstream slot. There are then more packets in the ONU's buffer than the ONU had asked to send in that slot. This situation may continue for several cycles until the data arrival rate drops.
 FNT Some protocols are for example, disclosed in Karlsson JM et al: "Adaptive Polling Schemes for an ATM bus with bursty arrivals", Computer Networks and ISDN systems; vol. 24, no. 1, Mar. 1, 1992, pages 93-103, x p000250094.
MAC protocols attempt to implement a control policy capable of coping with changes in demand for service by ONUs. PONs may carry many types of service traffic, from low bit rate synchronous services to variable high bit rate `data` services. Protocols attempt to allocate capacity fairly so that, for example, a low rate voice connection is not delayed, causing echo, by a demand for a high rate data service. With advanced PONs, using fibre amplifiers to increase the length, number of branches and data rate, the demands on the protocol are greater, being called upon to cope with potentially thousands of simultaneous calls, longer round-trip delays due to the greater reach of the PON and to be able to multiplex many different fixed and variable rate service types. Thus, the choice of MAC protocol may have a great influence on the performance of the network.